Explanation:
Either Lord Brahma or Lord Vishnu could not understand the real Divine form of Lord Siva. Here, Lord Siva is not treated as one of the Trinities, but Supreme Soul/Truth himself. The Trinities and other Devas have existence only depending on that Supreme Truth or even could enter into their own duties. Upanishad manthra “Bheeshmaasmadva pavathe Bheeshodethi surya” indicates this principle - Wind blows strongly fearing this; sun rises with the same kind of fear.
In the holy poetical works (Puranaas), the story of Lord Brahma and Lord Vishnu who argued about their own Superiority, once had to face a competition to determine the end point of an apparent infinite Luster pillar appeared before them. Lord Vishnu who went to the bottom side of the pillar could not find the terminal point and had to return admitting the failure. Lord Brahma, who went to the top side could not the find the terminal point as well. But he happened to see a Pandanus flower, which was descending and when enquired, it said it is coming from the top end point. Lord Brahma utilized this opportunity by not proceeding further upwards, but taking Pandanus flower, descended to meet Lord Vishnu and claimed that he saw the top end of Luster pillar, supported by the witness of Pandanus flower.
In fact, it was Lord Siva who appeared as Luster pillar. Since Lord Brahma lied, Lord Siva was furious and chopped his fifth head. (Lord Brahma had originally five heads, four facing the four different directions and one facing towards top. It was his head facing upwards, cut by Lord Siva). Due to this lack of morality, Lord Siva cursed Lord Brahma that he shall not be worshipped in any temples. That is why we cannot see any temples having main idol as Lord Brahma. Lord Siva also condemned Pandanus flower that, let no devotee use it for any kind of worship purposes.
From this story and similar others in ancient classics, it is clear that only Supreme Truth has Pure existence. The existence of Devas, human beings and multitude of living beings is dependent on it. It is impossible to understand the varieties and multiple forms of Supreme Truth by Trinities, Divine Beings or human beings. This is the message embedded in the line “Poomalaronum thirumaalum aarum Ponpoomeni kandillennu” as brightly illustrated by the Guru.